Abstract
Infrared spectra of species adsorbed on supported metal catalysts are subject to dipole coupling effects which have been extensively studied on single crystal substrates. Coupling interactions influence spectra in three ways: frequency shifts, changes in absolute molecular absorption intensities, and transfers of intensity from one absorption band to another. The magnitude of frequency shifts depends on the size of the adlayer, and may consequently vary with the size of the catalyst particles, while depolarization effects can reduce molecular absorption intensities by a factor of three or four at high adsorbate densities. The most striking influence of coupling on spectra from supported catalysts, however, arises from intensity transfers between molecules adsorbed on dissimilar sites. At high surface coverages the apparent importance of a small number of defect sites can be greatly enhanced by this effect, and it is even possible for a strongly bound minority species to be wrongly identified as a weakly bound majority one. Such errors can be avoided if spectra from isotopic mixtures are recorded.
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